Compression Ratio Calculator
Calculate the compression ratio of an internal combustion engine based on cylinder bore, stroke, head gasket thickness, deck height, piston dome/dish volume, and combustion chamber volume.
Published by:
isCalculator Editorial Team
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How to Use the Compression Ratio Calculator
This calculator computes the static compression ratio (CR) of an internal combustion engine using metric units (mm for dimensions, cc for volumes). Follow these steps:
- Enter bore and stroke (both in mm). These define the cylinder dimensions.
- Set head gasket thickness (mm). Default is 1.5 mm; use 0 if no gasket.
- Input deck height (mm). Positive if piston is below deck, negative if above. Default is 0.
- Enter piston volume (cc). Use positive for dome (+), negative for dish (-). Default is 0.
- Enter combustion chamber volume (cc). This must be a positive number.
- Click "Calculate Compression Ratio".
Results: The compression ratio (e.g., 10.5:1) is displayed along with swept volume and clearance volume. The formula used is: CR = (Swept Volume + Clearance Volume) / Clearance Volume.
Note: All inputs must be valid numbers. Bore, stroke, and chamber volume must be positive. Clearance volume must be greater than zero. Errors are highlighted in red.
Previous Results
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Frequently Asked Questions
Compression ratio (CR) is the ratio of the volume of the cylinder and combustion chamber when the piston is at bottom dead center (BDC) to the volume when the piston is at top dead center (TDC). It indicates how much the air-fuel mixture is compressed.
Combustion chamber volume is typically measured in cubic centimeters (cc) using a burette and a plexiglass plate with a hole (cc'ing method). It can also be found in engine specifications.
Typical street engines have CR between 8:1 and 10:1. Higher ratios (10:1 to 12:1) require higher octane fuel. Turbocharged engines often run lower CR (8:1 to 9:1) to avoid detonation.